1. Field of the Invention
This invention relates to the field of porous materials, and in particular to a method of making an open cell metal or ceramic material.
2. Description of Related Art
Porous metal or ceramic materials are currently used for the fabrication of devices such as filters, heat exchangers, sound absorbers, electrochemical cathodes, fuel cells, catalyst supports, fluid treatment units, lightweight structures and biomaterials. The structures (open/closed porosity, pore size distribution and shape, density) and properties (permeability, thermal, electrochemical and mechanical properties) required greatly depend on the application. Closed porosity is generally sought for lightweight structure while open porosity is particularly sought when surface exchange are involved or when permeability or pore connectivity is required.
Different approaches have been proposed for the fabrication of such porous materials. Good reviews of manufacturing methods and characterization of porous metal or ceramic material are given in Porous and Cellular Materials for Structural Applications, Materials Research Society Symposium Proceedings Vol. 521, Apr. 13-15, 1998, San Francisco, D.S. Schwartz et al. Ed., Materials Research Society; Metal Foams, Fraunhofer USA Metal Foam Symposium, J. Banhart and H. Eifert, ed. Stanton, Delaware, Oct. 7-8, 1998, and R. Soria, xe2x80x9cOverview on Industrial Membranesxe2x80x9d, Catalysis Today, 25, pp. 285-290 (1995).
Deposition techniques have been used for the fabrication of metal foam. U.S. Pat. No. 4,251,603 and Japanese Patent Laid-Open Patent Application No. 5-6763 describe processes consisting of plating a sponge-like resin and then burning the resin to obtain a metal foam. Deposition may also be done from salts (U.S. Pat. No. 5,296,261) or gas (U.S. Pat. No. 4,957,543). Those processes provide low-density materials having open-cell porosity.
Direct foaming of melts is described in various patents, for example, U.S. Pat. Nos. 3,794,481; 4,713,277; 4,973,358; 5,112,696 and WIPO publications Nos. WO 91/03578, WO 92/03583, WO 94/172218, WO 91/01387, WO 91/19823, WO 94/09931, WO 92/21457, European Pat. No. 0 210803 and Norwegian Pat. PCT/NO90/00115. In techniques described in these patents, foaming is carried out by blowing gases in the melt or adding chemical foaming agents such as titanium hydride which release gas when heated and creates bubble in the melt. Melt viscosity is generally adjusted using additives such as silicon carbide, aluminum oxide, magnesium oxide or calcium. These processes provide foams with good mechanical properties. The resulting foams have closed porosity.
An alternative approach to produce metal foams from liquid metals is the solid-gas eutectic solidification (Gasars) method such as described in U.S. Pat. No. 5,181,549. The method utilizes an enclosed vessel in which a base material is melted. A gas, whose solubility in the base material decreases with decreasing temperature and increases with increasing pressure, is dissolved into the base material. The metal is then cooled at a predetermined pressure to precipitate the gas and form pores in the solidified material.
Investment casting is also known for the fabrication of metal foams. A polymer foam having open pores is filled with a slurry of heat resistant material. The impregnated foam is then dried and heated at moderate temperature to eliminate the polymer. The resulting heat resistant porous structure is then impregnated with a liquid metal. After solidification, the mold is removed using pressurized water. The final metal foam has the original polymer foam structure. The material has good mechanical properties and large interconnected porosity.
Powder metallurgy has also been extensively used to produce porous materials using different approaches. Some techniques use a combination of solid and liquid state processing to produce metal foam from powders. U.S. Pat. No. 3,087,807 by B. C. Allen, M. C. Mote and A. M. Sabroff describes a method to produce lightweight, porous metal structure comprising the step of compacting a mixture containing aluminum powder and a foaming agent, selected from the group consisting of calcium carbonate, zirconium hydride and titanium hydride, which releases a substantial amount of gas at about the melting temperature of aluminum, extruding the resulting compact below the melting point of aluminum to form a rod, progressively heating the extruded rod to at least the melting temperature of aluminum to produce a foam, and rapidly cooling the resulting foamed material to form a lightweight porous structure having a uniform close cell porosity and density of about 0.45 to 0.58 g/cm3.
A modified approach, described in U.S. Pat. No. 5,151,246 by J. Baumeister and H. Schrader, consists of manufacturing foamable metal bodies in which a metal powder and a foaming agent powder is hot-compacted to a semi-finished product at a temperature at which the joining of the metal powder particles takes place primarily by diffusion and at a pressure which is sufficiently high to hinder the decomposition of the foaming agent in such fashion that the metal particles form a solid bond with one another and constitute a gas-tight seal for the gas particles of the foaming agent. The foamable metal body can also be produced by rolling.
An approach described in U.S. Pat. No. 5,865,237 by F. Schorghuber, F. Simancik and E. Hartl involves providing compacts of a powder of a metal to be foamed and a gas-evolving foaming agent; heating a volume of said compacts in a heatable chamber communicating with a mold having a mold cavity of a shape complementary to the casting to be made which, upon complete foaming, corresponds at least to the volume of said mold cavity, the heating of said compacts being sufficient to at least partially foam the metal of said powder; while said metal of said powder is being foamed in said chamber, forcing the entire contents of said chamber, formed by foaming of said compacts, into said mold cavity; and permitting residual foaming of said contents in said cavity to distribute the foaming metal to all parts of said cavity and produce a foamed metal body conforming completely to said cavity.
Techniques involving the deposition of powders on polymer medium (foams or granules) have also been developed. Those techniques consist in deposing metal or ceramic particles on a polymer and burning the polymer to obtain porous metal or ceramic materials. U.S. Pat. No. 5,640,669 by K. Harada, M. Ishii, K. Watanabe and S. Yamanaka describes a process for preparing a metal porous body having a three-dimensional network structure by deposing a layer comprising Cu, a Cu alloy, or a precursor thereof on a skeleton composed of a porous resin body having a three-dimensional network; heat-treating the resin body with the layer formed thereon to remove the heat-decomposable organic component, thereby forming a porous metal skeleton of Cu or a Cu alloy. U.S. Pat. No. 5,759,400 by C. E. Fanning describes the fabrication of metal foams by cutting a polyethylene foam to form a substrate having a desired size and shape, submerging the polyethylene substrate into a solvent for a period of time effective to provide a substrate with a tacky surface, coating the tacky surface of the polyethylene with a slurry of copper powders admixed with a binder, drying the impregnated polyethylene foam, burning the polyethylene in a furnace to produce a foam structure consisting of copper and sintering the final product to obtain a rigid structure.
U.S. Pat. No. 5,881,353 by Y. Kamigata, T. Yoshida, K. Susa, T. Uchida and H. Hiratsuka discloses a method for producing a porous body with high porosity by coating a resin foam, such as urethane foam, with an adhesive to impart stickiness to the surface of the foam, and thereafter a powder such as copper oxide powder is applied thereto, followed by heating to remove the substrate and sinter the powder. Thus, a porous body to which the pattern of the base material has been transferred is produced. The powder may be appropriately selected to obtain porous bodies having a great strength, without limitations on materials.
Methods for preparing porous hollow spheres and sponge-like particles are described in U.S. Pat. No. 4,775,598 by M. Jaeckel. Such porous hollow spheres could be used to produce porous materials. The process for making hollow spherical particles, comprising the steps of providing metallized lightweight spherical bodies from cores of a foamed polymer with a metal coating of a thickness of 5 to 20 microns; coating said metallized lightweight spherical bodies with a dispersion of at least one particulate material selected from the group which consists of metals, metal oxides, ceramics and refractories to a dispersion coating thickness of 15 to 500 microns; drying the dispersion coating on said metallized lightweight spherical bodies to form a dry layer of said material thereon; heating said metallized lightweight spherical bodies with said dry layer of said material thereon to a temperature of about 400xc2x0 C. to pyrolyze said polymer cores and form hollow bodies essentially consisting of said metal coatings and said dry layers of said material thereon; and subjecting said hollow bodies essentially consisting of said metal coatings and said dry layers of said material thereon to a sintering temperature of 900xc2x0 C. to 1400xc2x0 C. for a period sufficient to sinter the material of the respective layer and the respective layer to the respective metallic coating, thereby forming hollow spherical particles.
Sintering of freely poured powder, as described in V. M. Kaptsevich et al. xe2x80x9cInfluence of the Morphology of the Original Powder on the Properties of Porous Materialsxe2x80x9d, Sov. Powder Metall. Met. Ceram., 29 (4), pp. 308-313 (1990), or in G. Paruthimal et al. xe2x80x9cAnalysis of Porous Iron Electrodes by Scanning Electron Microscopyxe2x80x9d, B. Electrochem., 5 (2), pp. 99-105 (1986), has also been used to produce components having high porosity. Powder mixtures may or may not contain additives such as pore formers or foaming agents. C. Solaiyan et al., xe2x80x9cPreparation and Characterization of Porous Electrodes from Nickel Powder for Fuel Cellsxe2x80x9d, Indian J. of Chemical Technology, 6, pp.48-54 (1999), describe processes for the preparation of porous electrodes by compacting layers of mixture composed of nickel particles admixed with 5-20 wt. % pore formers elements such as Na2CO3, KCl, NH4HCO3 and naphthalene. C. Stiller et al., xe2x80x9cManufacturing and Characterization of Low Density Titanium Partsxe2x80x9d, Proc. of 1998 Powder Metallurgy World Congress and Exhibition, Granada, Spain, Ed. by EPMA, Vol.5, pp. 189-194, describe a process for manufacturing porous titanium parts by compacting a mixture of titanium powder with a space holder material (carbamide). Materials produced using those techniques have open porosity. Depending on the powder used, fine pore size distribution may be obtained. However, structures with very low density are more difficult to produce.
Slurries have also been extensively used to produce porous materials. The slurries contain metal or ceramic particles, a liquid medium and optionally surface-active agents, binders, gelling agents, stabilizing agents and foaming agents. Optionally, gas may be injected in the slurry. Slurries are cast or poured in a mould. The resulting product is dried at moderate temperature, debinded and sintered to provide strength.
U.S. Pat. No. 5,132,080 by L. B. Pfeil describes an horizontal process for the production of a continuous porous metal strip which comprises forming a slurry of metal powders in a liquid medium, depositing said slurry via leveling means on a flat horizontal moving surface in a slurry layer of uniform thickness, immediately thereafter horizontally passing said slurry layer through an evaporating zone, a calender zone, and then through a sintering zone whereby the liquid medium is substantially removed and cohesion between the metal particles is subsequently obtained, and thereafter continuously separating the cohesive porous strip thus formed from the moving surface.
U.S. Pat. No. 4,430,294 by V. A. Tracey describes a slurry process for the production of porous nickel bodies characterized by high strength, residual carbon content below about 0.08 wt. %, and a porosity exceeding 75 v %, the process consisting essentially of providing a carbonyl nickel powder, forming a nickel-carbon mixture by adding carbon particles to the nickel powder in an amount sufficient to raise the carbon content thereof to about 0.35-2 wt. % carbon, the particle size of the carbon no greater than the particle size of the nickel powder, forming a green body from the mixture, sintering the body in a reducing atmosphere, the temperature of the reducing atmosphere between 750xc2x0 C. and 1050xc2x0 C., the reducing atmosphere consisting essentially of hydrogen, nitrogen and 15-30 v % carbonaceous gas, for the time necessary to maintain the porosity of the body above 75 v %. The specimens prepared in the examples were fabricated in aqueous solution of methyl cellulose containing a defoaming agent.
U.S. Pat. No. 3,796,565 by H. A. Hancock and D. J. I. Evans describes a process for making a porous nickel plate including the steps of providing a starting material composed of nickel powder; adjusting the content of nickel oxide in said starting material to about 0.7 to about 1.4 percent by weight; dispersing the so-adjusted starting material in a volatile liquid to form a slurry; heating the slurry to a temperature below sintering temperature but sufficient to evaporate the volatile liquid fraction thereof; and sintering said heated slurry in a reducing atmosphere.
U.S. Pat. No. 4,225,346 by C. D. Helliker and T. D. O""Sullivan describes a process for making porous nickel bodies of various shapes by forming, debinding and sintering a gel mixture, the gel mixture being prepared by mixing together an aqueous solution of modified cellulose ether which gels on heating and a metal powder consisting essentially of nickel and heating this mixture to a temperature between 50 and 120xc2x0 C. in order to achieve gelling.
U.S. Pat. No. 3,897,221 by I. O. Salyer and R. T. Jefferson discloses a method for preparing porous metal structures by forming a polyurethane structure containing powdered metal, said structure being formed by mixing metal powder in a solution containing the polyurethane forming reactants, polymerizing the mixture in place without stirring after onset of gelation; removing the polyurethane, preferably by heating in air at a temperature below the sintering temperature of the metal, and sintering the remaining porous metal or metal oxide structure.
U.S. Pat. No. 4,569,821 by G. Duperray and M. Hilaire discloses a method for preparing a porous metal body, the method comprising the steps of adding a surface active agent and a gelling agent to water; agitating the mixture to produce a foam; incorporating metal powder into the foam to obtain a suspension of the metal in the foam; adding a stabilizing agent to the foam, stabilizing agent being a material that polymerizes upon contact with water; shaping the stabilized foam suspension; allowing the shaped and stabilized foam suspension to set; and heating the set foam to a temperature high enough to burn the organic material therein and to sinter the suspended metal powder.
U.S. Pat. No. 5,848,351 by K. Hoshino, Y. Mayuzumi, T. Kohno and N. Komada discloses a porous metallic material produced by preparing a foamable slurry containing for example a metal powder, a water-soluble resin binder, a water insoluble hydrocarbon organic solvent (foaming agent), a surfactant and water, forming the foamable slurry, drying the formed product, preferably after foaming, and finally heating the dry formed product to eliminate the resin binder and sinter the material. The obtained material has a low-density three-dimensional network structure, which is composed entirely of a sintered metal powder. The resulting material has high specific surface area.
U.S. Pat. No. 3,833,386 by L. L. Wood, P. Messina and K. C. Frisch discloses a method for the preparation of ceramic foam structures prepared by reacting an isocyanate-capped polyoxyethylene polyol reactant with large amount of an aqueous reactant containing finely divided sinterable ceramic material. The resultant foams having the sinterable ceramic material dispersed thereon are heat-treated under firing conditions to decompose the carrier foam and sinter the ceramic particles. The resulting material is a rigid ceramic foam structure.
U.S. Pat. No. 5,213,612 by W. Minnear and B. P. Bewlay discloses a method for forming a porous body of a metal from the group consisting of molybdenum, molybdenum alloys tungsten, tungsten alloys or mixtures thereof comprising the foaming of an aqueous slurry of a sinterable metal powder and a foaming agent in a volume ratio of about 0.6 to 3.5:1 respectively, to form a foam having the metal powder dispersed therein; drying and heating the foam to decompose the foam and sinter the metal powder in a reducing atmosphere to promote interparticle diffusion and bonding.
French patent No. 1,573,864 discloses a method of making a foam or cellular structure. The process described in this patent employs two different types of agent: foaming agents and spacing agents. The effects of these two agents are quite different. Foaming agents create a gaseous volume expansion to create a foam within a liquid. Spacing agents provide bulk, which is subsequently removed to leave voids where the spacing agents were present.
In the first eight examples of the French patent, foaming agents are employed to create a foam from a slurry containing an inorganic powder, a liquid resin and a foaming agent. Example 9 of the French patent discloses a process for making a porous structure wherein a silicium powder is mixed with a liquid binder. The resulting mixture is reduced to powder, mixed with camphor and then compacted into blocks. The camphor is then vaporized to remove it from the blocks and leave voids in its place. In this case the camphor acts purely as a spacing agent since it merely creates bulk, which is removed to create the voids between the particles and no foaming occurs.
It has been found surprisingly that open cell materials with unique microstructure can be obtained by using a dry powder mixture. Such as mixture can be made easily flowable. Generally, it would be thought that a powder would not be suitable for foaming because the released gases would tend to flow around the particles and escape without creating a foam.
Contrary to expectations it has been discovered that foaming can be made to occur starting from a powder mixture because during the thermal treatment the binder melts and flows around the particles, thereby closing the pores before foaming starts to occur.
Accordingly the present invention provides a method of making a porous body, comprising the steps of providing a dry flowable powder mixture comprising 10-90 wt. % of sinterable particles, 10-90 wt. % of binder, and 0.25-5 wt. % of foaming agent; shaping said flowable powder mixture into a predetermined form; heating the product to melt the binder while inducing foaming in the mixture; heating the foamed structure to pyrolyze said binder; and sintering the debinded material to form a solid low-density open-cell foam. Optionally, pressure may be applied to the powder mixture before or during the heating of the powder mixture.
The foaming agent may be a solid, but it can also be a liquid embedded or in solution in the binder, a gas in solution in the binder, or a mixture of two or more foaming such agents. As long as they are embedded or in solution in the binder, the final mixture will remain a dry flowable mixture.
The sinterable particles may be coated particles, metallic particles, ceramic particles, or a mixture of metallic, ceramic and/or coated particles.
Optionally, a cross-linking agent may be added to the dry powder mixture to cure the binder and improve the mechanical strength of the foamed structure before debinding.
Optionally, 0.01 wt. % to 5 wt. %, but preferably between 0.05 wt. % to 5 wt. % additional binder may be added to minimize segregation and dusting and improve the flowability of the dry powder mixture.
In accordance with the invention, the dry powder mixture is preferably heated in a stepwise fashion during the respective foaming, debinding and sintering processing steps. Heating rates and temperature plateaux depend on the powder composition, forming conditions, microstructure and properties required and the applications. Typically, the temperature used to melt the binder and foam the structure ranges from 40xc2x0 C. to 300xc2x0 C., but preferably lies between 75xc2x0 C. and 200xc2x0 C. Optionally, the heat treatment during the foaming steps may induce some consolidation of the binder by cross-linking if for instance thermoset binder or thermoplastic binder with cross-linking agent are used.